Small code simplifications and add tests

[cmdcolin]

Bug fixes

• Added input validation for empty/whitespace search strings to prevent undefined.toLowerCase() error
• Added filter for empty parts when parsing line data

Refactoring

• Restructured search() to use nested if statements instead of early returns
• Simplified termination logic in search loop - combined conditions to avoid duplicate startsWith calls
• Renamed _getBuffer to getBuffer (underscore unnecessary with private modifier)
• Removed dead code (len < 0 check that could never be true)
• Fixed variable shadowing in sum() function (sum → total)
• Simplified filter syntax (!!f → f)

Tests

• Added edge case tests for empty string, whitespace, and leading/trailing space handling
• Added unit tests for dedupe(), sum(), and concatUint8Array() utilities